The European Union currently consists of 28 member states. One of the characteristics of the Union, is freedom to provide services within all of the EU, including freedom of movement of workers to provide such services, also called ‘posting’. However, that freedom is restricted. In general, this means pre-posting registration requirements.
If you want to post workers in another EU country (have your employees work on project in another member state), you need to take so-called ‘posting rules’ into account. In essence, this means that your company needs to take the same conditions and terms (e.g. salary, rest working hours, holidays) into account as those which are in force in the host country.
Summarily, if you post workers in Spain, you need to pay them (at least) Spanish level wages, they are entitled to Spanish paid holidays and rest periods, etceteras. Another element of posting is pre-posting registration of employees in the host country, so prior to commencement of activities; pre-registration requirements must be followed and may be heavily sanctioned. This is sometimes is seen as a barrier to entry and contradictory to free movement of workers.